Direct Current (DC) fans are predominantly used to cool electronic equipment. The fans are typically brushless in design, to reduce cost and improve reliability. The commutation of the fan generates a current pulse on the power source. This current pulse varies in amplitude and frequency, proportional to the fan speed. Typically, the current pulse ranges from 0 Hz to 500 Hz, and may also exhibit harmonics well into the 10 kHz to 20 kHz range.
Regulatory agencies are now requiring the conducted noise in the voice band to be below low levels, for example 9+10 logIc (dBrnC), where Ic is the maximum measured input current on the power port of the telecommunications equipment. Equipment that is designed to measure analog signals with C-Message weighting is specified to measure the noise. Typically, this is performed using a Transmission Impairment Measurement Set (TIMS) that is used for measurements on Plain Old Telephone Systems (POTS). The term dBrnC is a decibel relative measurement with C-Message weighting, and is specified as dBrnC=dBm+90. Thus, as an example, an input current of 2A results in a limit of 12 dBrnC. Applying the conversion formula results in an absolute value of 12 dBrnC−90=−78 dBm in the voice band range.
Attenuating noise down to this low of a level can be achieved using a passive low-pass filter consisting of inductors, capacitors, and resistors on the power supply to the fan(s) or on the power supply port of the equipment cooled by the fan(s). The size of the filter components can increase significantly, depending on the current level required on the power port. Passive low-pass filters are widely used technology having moderate cost. However, using low pass filters causes significant pressure on the volume available to house all the components, depending on the level of attenuation and current rating.
Another method used to reduce the noise level is to use Pulse Width Modulation (PWM) on the fan power input. In the case where multiple fans are used in a system, the fans' PWM drives are driven out of phase with each other to minimize the additive current pulses. U.S. published patent application 2009/0278484 A1 dated Nov. 12, 2009 shows one example of incorporating this method.